Abstract: Clarifying the tectonic affinity of the southern continental margin of the South China Sea (SCS) and constraining when it was part of the southeastern continental margin of South China Block (SCB) is critical for reconstructing the Mesozoic–Cenozoic paleogeography of Southeast Asia. Furthermore, prior to the opening of the SCS, the proto–South China Sea (PSCS) is thought to have been located along the southeastern continental margin of the SCB. However, the timing of initial spreading and the geodynamic mechanism responsible for the opening of the PSCS remain poorly constrained. To solve these issues, we conducted geochronological, petrographic, and geochemical analyses on igneous rocks from the Well NK-1 drilled in the Nansha Block of the southern SCS margin in this study. Our results identify three distinct stages of Mesozoic magmatism in the Nansha Block: Late Triassic dacite (~218–217 Ma), Early Jurassic gabbro-diorite (~177 Ma), and Early Cretaceous diabase (~124 Ma). Magmatic events of comparable age and composition are also widely documented within the interior of the SCB. Together with geophysical and paleontological evidence, these findings indicate that the southern SCS margin—represented by the Nansha Block—was part of the southeastern SCB margin at least since the Triassic. These three magmatic episodes collectively record the progressive Mesozoic tectonic evolution along the southeastern SCB margin. The Late Triassic dacites exhibit geochemical characteristics of A2-type granites, implying formation in an intracontinental extensional regime driven by far-field paleo-Pacific subduction. The Early Jurassic gabbro-diorites show geochemical affinities to continental arc basalts, reflecting subduction-related continental arc magmatism associated with the paleo-Pacific plate. The Early Cretaceous diabases display geochemical features typical of back-arc basin basalts, consistent with emplacement during a back-arc extensional stage induced by paleo-Pacific slab rollback. Moreover, the Early Cretaceous diabases from Well NK-1 represent continental-margin extensional magmatism associated with the initial opening of the PSCS. Its geochemical characteristics, together with those of the Early Cretaceous remnants of the PSCS oceanic crust, consistently exhibit features of back-arc basin basalts, indicating that the PSCS developed from a back-arc basin formed by slab rollback of the paleo-Pacific plate during the Early Cretaceous. This study provides insights into reconstructing the Mesozoic tectonic evolution of the southern SCS margin and Southeast Asia.

Key words: Well NK–1, Southern South China Sea, Proto–South China Sea, South China Block, Paleo–Pacific plate